U.S. patent number 6,670,777 [Application Number 10/185,765] was granted by the patent office on 2003-12-30 for ignition system and method.
This patent grant is currently assigned to Woodward Governor Company. Invention is credited to Jeffrey B. Barrett, David C. Petruska, Mark R. Woolston.
United States Patent |
6,670,777 |
Petruska , et al. |
December 30, 2003 |
Ignition system and method
Abstract
An ignition system for an engine includes an exciter circuit for
use with an igniter, the exciter circuit having a step-up
transformer the utilizes a relatively low voltage in its primary to
produce a high voltage pulse that is applied to the igniter to
create ionization and breakdown. The system also utilizes a low
voltage high energy circuit to provide high current energy to the
igniter after initial breakdown and during the plasma arc phase.
The high energy circuit is decoupled from the step-up transformer
so that high current is conducted through a bypass diode rather
than through the transformer.
Inventors: |
Petruska; David C. (Fort
Collins, CO), Barrett; Jeffrey B. (Bolton, MA), Woolston;
Mark R. (Bolton, MA) |
Assignee: |
Woodward Governor Company
(Rockford, IL)
|
Family
ID: |
29735242 |
Appl.
No.: |
10/185,765 |
Filed: |
June 28, 2002 |
Current U.S.
Class: |
315/209CD;
123/598; 123/606; 361/253 |
Current CPC
Class: |
F02P
3/0876 (20130101); F02P 17/12 (20130101) |
Current International
Class: |
F02P
3/08 (20060101); F02P 17/12 (20060101); F02P
3/00 (20060101); H05B 037/02 () |
Field of
Search: |
;315/29CD,241R,247,278,279,225 ;361/253,257
;123/598,606,607,620,635,653 |
References Cited
[Referenced By]
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|
Primary Examiner: Nguyen; Hoang V.
Attorney, Agent or Firm: Greer, Burns & Crain, Ltd.
Claims
What is claimed is:
1. An ignition system comprising: an igniter for creating a spark;
a step-up transformer having a primary winding and a secondary
winding the secondary winding being operably connected to one
terminal of said igniter, a first energy storage device for
providing a first amount of energy at a first voltage level, one
terminal of said device being connected to a second terminal of
said igniter; a first switch connected to said first energy storage
device for controlling the release of energy therefrom, said first
switch being connected to said one terminal of said igniter through
said secondary winding of said transformer; a second energy storage
device for releasing a second amount of energy at a second voltage
level to said primary winding of said transformer; a second switch
connected in circuit with said primary winding for controlling the
release of energy from said second energy storage device through
said primary winding of said transformer, said energy being
transformed to a stepped-up third voltage level and applied to said
igniter when said second switch is triggered into conduction; an
electrical bypass connected to said first switch and said one
terminal of said igniter in parallel with said secondary winding of
said transformer, thereby permitting said first amount of energy to
bypass said secondary winding of said transformer and be applied to
said one terminal of said igniter; and, a charging circuit for
charging said first and second energy storage devices; and, a
controller for triggering said first and second switches.
2. An ignition system as defined in claim 1 wherein said controller
triggers said first switch and triggers said second switch a
predetermined time after it triggers said first switch.
3. An ignition system as defined in claim 2 wherein said
predetermined time is within the range of approximately 0.1
microseconds to 100 microseconds.
4. An ignition system as defined in claim 1 wherein said second
switch is a silicon controlled rectifier (SCR).
5. An ignition system as defined in claim 1 wherein said first
switch comprises a pair of silicon controlled rectifiers (SCR's)
connected in parallel to one another.
6. An ignition system as defined in claim 5 further comprising a
saturable reactor connected in series to each SCR of said SCR pair,
said reactor limiting the current flow through the SCR for a
predetermined time duration to protect each SCR from damage while
it is triggered into conduction.
7. An ignition system as defined in claim 6 wherein said
predetermined time duration is approximately 1-10 microseconds.
8. An ignition system as defined in claim 1 wherein said second
energy storage device is a capacitor, said second amount of energy
is less than 2 Joules and said second voltage level is less than
1000 VDC.
9. An ignition system as defined in claim 1 wherein said first
energy storage device is one or more capacitors, said first amount
of energy is less than 20 Joules and said first voltage level is
less than 2000 VDC.
10. An ignition system as defined in claim 1 wherein said third
stepped-up voltage level is to a level required for ionization.
11. An ignition system as defined in claim 1 wherein said bypass
comprises one or more diodes.
12. An ignition system as defined in claim 1 further comprising a
negative clamping diode connected in parallel with said first
energy storage device with its anode connected to said second
terminal of said igniter.
13. An ignition circuit for use with an igniter for creating a
spark, comprising: transformer means having a primary winding and a
secondary winding and being configured to step-up a first voltage
level applied to said primary winding to a higher second voltage
level, the secondary winding being electrically connected to one
terminal of the igniter; first storage means for providing a first
amount of energy at a third voltage level, one terminal of said
storage means being connected to a second terminal of the igniter;
a first switch for controlling the release of energy from said
first storage means; second storage means for releasing a second
amount of energy at said first voltage level to said primary
winding of said transformer; a second switch for controlling the
release of energy from said second storage means, said energy being
transformed to said second voltage level and applied to the igniter
when said second switch is triggered into conduction; bypass means
connected to said first switch and said one terminal of the igniter
in parallel with said secondary winding of said transformer means,
thereby permitting said first amount of energy to bypass said
secondary winding of said transformer means and be applied to said
one terminal of the igniter; a low voltage bus for powering
components for operating said circuit; including charging said
first and second energy storage devices; and, a controller for
triggering said first switch followed by triggering said second
switch.
14. An ignition circuit as defined in claim 13 wherein said low
voltage bus has a voltage level less than approximately 2000
VDC.
15. An ignition circuit as defined in claim 13 wherein said second
storage means is a capacitor, said second amount of energy is less
than 2 Joules and said first voltage level is less than 1000
VDC.
16. An ignition circuit as defined in claim 13 wherein said first
energy storage device comprises one or more capacitors, said first
amount of energy is less than 20 Joules and said third voltage
level is less than 2000 VDC.
17. An ignition circuit as defined in claim 13 wherein said second
stepped-up voltage level is the level required for igniter
ionization.
18. A method of igniting fuel in an engine comprising the steps of:
charging a first energy storage device to a first predetermined
energy level utilizing a first predetermined voltage; charging a
second energy storage device to a second predetermined energy level
utilizing a second predetermined voltage; triggering a first switch
at a first time, the first switch being connected in series with
the first energy storage device and one or more bypass diodes, the
diodes being connected in parallel with a secondary winding of a
step-up transformer; and, triggering a second switch connected in
series with said second energy storage device and a primary winding
of said transformer into conduction at a second time later than
said first time and applying the energy from said second energy
storage device to the primary of the step-up transformer, the
energy applied to the primary winding producing a stepped-up
voltage in the secondary winding of said transformer; applying the
stepped-up voltage to a sparking generating device to create a
spark for the purpose of igniting fuel in the engine; and, applying
the energy from said first energy storage device to said spark
generating device.
19. A method as defined in claim 18 wherein said second time is
within the range of approximately 0.1 microseconds to 100
microseconds later than said first time.
20. A method as defined in claim 18 wherein said second energy
storage device is a capacitor, said second predetermined energy
level is less than 2 Joules and said second predetermined voltage
is less than 1000 VDC.
21. A method as defined in claim 18 wherein said first energy
storage device is one or more capacitors, said first predetermined
energy level is less than 20 Joules and said first predetermined
voltage is less than 2000 VDC.
22. A method as defined in claim 18 wherein said stepped-up voltage
is a voltage level required for ionization and is up to
approximately 40,000 VDC.
23. A method of generating a spark utilizing a circuit that has a
step-up transformer with a primary winding and a secondary winding,
the circuit having a primary side and a secondary side, the primary
side including a low energy storage device and a primary side
switch, the secondary side having a spark generating device and
including a high energy storage device connected to the spark
generating device through a secondary side switch and a bypass
means connected in parallel to the secondary winding of the
transformer, and a charging means for charging the high and low
energy storage devices, comprising the steps of: charging the high
and low energy storage devices to their respective energy levels at
a respective relatively low voltages within a predetermined range;
triggering the secondary side switch at a first time; triggering
the primary side switch into conduction at a second time later than
the first time and applying the energy from said low energy storage
device to the primary winding, the energy applied to the primary
winding producing a stepped-up voltage in the secondary winding of
the transformer; applying the stepped-up voltage to the spark
generating device to create a spark; applying the energy from said
high energy storage device to the spark generating device through
the secondary side switch and the bypass means.
24. A method as defined in claim 23 wherein said second time is
within the range of approximately 0.1 microseconds to 100
microseconds later than said first time.
25. A method as defined in claim 23 wherein said low energy storage
device is charged at a charging voltage of less than 1000 VDC to an
energy level of less than 2 Joules.
26. A method as defined in claim 23 wherein said high energy
storage device is charged at a charging voltage of less than 2000
VDC to an energy level of less than 20 Joules.
27. A method as defined in claim 23 wherein said stepped-up voltage
is a voltage level sufficient for ionization.
28. A method of utilizing an igniter circuit that has a step-up
transformer with a primary winding and a secondary winding, the
circuit having a primary side and a secondary side, the primary
side including means for applying energy to the primary winding,
the secondary side being operably connected to an igniter in the
engine and including a high energy storage device connected to the
igniter through a secondary side switch and a bypass means
connected in parallel to the secondary winding of the transformer,
and a charging means for charging the high energy storage device,
comprising the steps of: charging the high energy storage device to
its energy level at a relatively low voltage; triggering the
secondary side switch at a first time; applying energy to the
primary winding after triggering the secondary side switch, the
energy applied to the primary winding producing a stepped-up
voltage in the secondary winding of the transformer; applying the
stepped-up voltage to the igniter to create a spark for the purpose
of igniting fuel in the engine; applying the energy from said high
energy storage device to the igniter through the secondary side
switch and the bypass means.
29. An exciter circuit for use with an igniter for creating a spark
for igniting fuel in an engine; comprising: transformer means
having a primary winding and a secondary winding and being
configured to step-up a first voltage level applied to said primary
winding to a higher second voltage level, the secondary winding
being electrically connected to one terminal of the igniter; a high
energy storage means for providing a first amount of energy at a
low voltage level, one terminal of said storage means being
connected to a second terminal of the igniter; a switch for
controlling the release of energy from said high energy storage
means; means for selectively providing energy to said primary
winding of said transformer, said energy being transformed to said
second voltage level and applied to the igniter; bypass means
connected to said switch and said one terminal of the igniter in
parallel with said secondary winding of said transformer means,
thereby permitting said first amount of energy to bypass said
secondary winding of said transformer means and be applied to said
one terminal of the igniter; a controller for triggering said
switch followed by operating said energy providing means.
30. An exciter circuit as defined in claim 29 wherein said low
voltage energy level is below approximately 2000 VDC.
31. An energy discharge system having an output, said system
comprising: a step-up transformer having a primary winding and a
secondary winding, said secondary winding being connected to the
output, an energy storage device for providing high current energy
to the output; a switch for controlling the release of energy from
said energy storage device; an electrical bypass connected in
circuit to said switch and the output and in parallel with said
secondary winding of said transformer, thereby permitting said high
current energy to bypass said secondary winding of said transformer
and be applied to the output.
32. An energy discharge system as defined in claim 31 further
comprising a second energy storage device connected in circuit with
said primary winding of said transformer for supplying a second
amount of energy for application to said primary winding.
33. An energy discharge system as defined in claim 32 further
comprising a second switch connected in series with said primary
winding for applying said second amount of energy to said primary
winding.
34. An energy discharge system as defined in claim 33 further
including a controller for selectively operating said switch and
said second switch.
Description
BACKGROUND OF THE INVENTION
The present invention generally relates to ignition systems and
more particularly to such systems, as well as to an exciter circuit
and a method of igniting fuel.
Ignition systems for turbine engines as well as other applications
have been in use for decades and they continue to evolve with
changing technology. Recent developments have included the
incorporation and use of solid state semiconductor power switching
devices for releasing energy from an energy storage device for
generating a spark discharge for igniting fuel in a turbine engine,
for example. Such solid state devices are considered to be more
reliable than gas discharge tubes that had been previously employed
for decades. Because such systems often have to reliably operate in
severe environmental conditions that include significant
temperature and air pressure variations, and because reliability
and safety considerations are of paramount concern when the
ignition systems are used in aircraft engines, for example, such
systems must be carefully designed for effective and reliable
operation.
It has been generally consistent practice to design exciter
circuitry that is used in connection with an igniter plug to employ
a relatively high voltage bus, i.e., on the order of at least 2000
to 3000 volts, so that the igniter plug reliably produces a
sufficient spark during operation. Serious design consideration has
been given to not only producing a sufficient initial spark, but
also one that is sustained so that reliable ignition of the fuel
occurs in the engine, particularly in severe environmental
conditions. However, when a high voltage bus is utilized in the
design of the exciter circuit, the components that operate in the
circuit must be capable of withstanding the high voltage and
current loads that are experienced. For example, if a high energy
capacitor is utilized in an exciter circuit and its energy is
released by a silicon controlled rectifier (SCR) switch, such a
single SCR switch that can handle the high voltage and current
loading may be very expensive. Alternatively, a switch design may
be utilized which employs multiple SCR's connected in a more
complex circuit arrangement. More particularly, such high voltage
switching is often performed by multiple series connected SCR's
which must be very carefully matched and triggered during operation
or they will likely prematurely fail.
While such high voltage ignition systems not only experience the
problems associated with finding reliable and cost efficient
components that can be used in such a high voltage environment,
they also do not necessarily result in the most efficient ignition
current waveform of energy delivery to the igniter plug. Typically,
a wave shaping inductor is placed between the energy storage
capacitor and the igniter in order to increase the current duration
and decrease the peak current going to the igniter.
SUMMARY OF THE INVENTION
The present invention includes a preferred embodiment ignition
system for a turbine engine which includes an exciter circuit that
has a step-up transformer utilizing a relatively low voltage in its
primary to produce a high voltage pulse that is applied to an
igniter to create ionization and breakdown. The system also
utilizes a low voltage high energy circuit to provide high current
energy to the igniter after initial breakdown and during the plasma
arc phase. The high energy circuit is decoupled from the step-up
transformer so that high current is conducted through a bypass
rather than through the transformer. Moreover, the low voltage of
the high energy circuit allows for smaller, less expensive and more
robust semiconductors to be used as the high energy switch.
The exciter circuitry carefully times the release of energy from a
separate primary side capacitor to the step-up transformer relative
to the operation of the SCR switch that releases the energy from
the high energy capacitor, which desirably protects the high energy
SCR switch during generation of the high voltage pulse that is
applied to the igniter plug. The low voltage topology, which
utilizes very large capacitance for the high energy capacitors,
produces an ignition current waveform with longer duration and
lower peak current than traditional prior art systems of equivalent
stored energy. The lower peak currents place lower peak power
stresses on the exciter components, while the longer duration
ensures high energy delivery through the igniter plug to the
combustible air/fuel mixture.
In the preferred embodiment of the present invention, the high
capacitance (e.g., 75 .mu.F) associated with the low voltage system
(e.g., 650V) allows for increasing current durations in the
presence of increasing external resistance. The low capacitance
(e.g., 3.5 .mu.F) associated with a traditional high voltage (e.g.,
2800V) system typically requires the addition of a current
discharge wave shaping inductor which increases the current
duration while reducing the peak currents to reasonable levels.
Furthermore, a low capacitance, unipolar system utilizing a typical
wave shaping inductor exhibits decreasing current durations in the
presence of increasing external resistance. Thus, the energy
delivery in the presence of increasing external resistance is more
consistent with a low voltage system. Sources of external
resistance include the ignition lead, which connects the exciter
and igniter, along with the igniter and igniter extensions.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a preferred embodiment of a turbine
ignition system of the present invention;
FIG. 2 is a simplified electrical circuit schematic diagram of the
preferred embodiment of the present invention; and,
FIG. 3 is an electrical timing diagram illustrating aspects of the
operation of the preferred embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Broadly stated, the present invention is described and implemented
in a preferred embodiment that is particularly useful as an
ignition system for a turbine engine. However, it should be
appreciated that the invention described in this patent can be used
in a much broader context that is certainly not limited to an
ignition system for a turbine engine. The present invention
certainly extends to and can be used more generally as an energy
discharge device or system that provides energy to an output that
could be as diverse an application as for energizing a laser. The
invention may also be used as an ignition system for gas or oil
fired furnaces, internal and external combustion engines, including
piston engines, as well as turbine engines.
The preferred embodiment of the ignition system of the present
invention is shown in the block diagram of FIG. 1 and includes a
set of external connectors indicated generally at 10 for inputting
AC power to the system as well as for providing communications
between the system and other systems that may be utilized by a user
for diagnostic purposes or for the purposes of checking or
modifying software used in the operation of the system.
AC power is provided on lines 12 which are connected to input power
conditioning circuitry 14 that preferably comprises an EMI input
filter, fuses and an AC to DC conversion circuitry which outputs an
unregulated 24VDC power on lines 16 and 18. Line 16 is connected
(via a voltage divider) to a digital signal processor 20, but is
not used to drive it, and line 16 is also connected to a high
energy 650 flyback circuit 22 (figure needs to be corrected to read
"650" not "560". The digital signal processor or DSP 20 is
preferably a microcontroller or microprocessor and preferably has
several analog to digital converter inputs, including one where
line 16 is applied to the DSP 20 so that it can monitor the voltage
range during operation.
The conditioning circuitry 14 is preferably standard transformer
and rectification functionality that provides a relatively
uncontrolled 24VDC bus at output line 16 and it is not important
that the output voltage be controlled within a limited range. In
practice, the output can vary between 18 to 40 volts as a function
of the input AC voltage and also the load being drawn essentially
as a function of the operation of the high energy flyback circuit
22. The 24VDC power applied on line 18 powers a DC to DC converter
circuit 24 that provides a regulated output of 5 volts, and
unregulated outputs of 8 volts and 12 volts for powering logic
circuits and the DSP20. The AC to DC conversion circuitry 14 as
well as the DC to DC converter 24 are considered conventional and
are therefore not shown in detail.
The system includes a temperature sensor 26 that provides a signal
to the DSP 20 for the purpose of monitoring the operation of the
system. When the temperature of the circuit boards in which the
circuitry is implemented reaches very high temperatures, the DSP 20
detects that and reduces the frequency of sparks being generated by
the system. In this regard, it should be understood that heat is
generated in proportion to the operation of the circuit and the
more often the system fires, the more heat is generated in the
circuit module. For example, if the system fires at a nominal 1.8
Hz frequency at an ambient temperature of 85.degree., when the
ambient temperature exceeds 100.degree., the firing rate may be
reduced to 1 Hz. It should also be understood that such frequency
variations as well as the values which are used to change the
firing rate may be programmed in the DSP 20.
The system also preferably includes a fault relay 30 that is
connected to the DSP 20 by line 32 and it has an output line 34
which may extend to other circuitry that may be used to control the
operation of the turbine engine itself. The fault relay 30 may be
triggered when the DSP senses through its inputs that something may
be wrong with the overall operation of the system. It provides a
state signal that can be employed by a user to provide further
signals or to control the operation of the turbine engine
itself.
An RS 232 module 36 is connected to the DSP via line 38 and it has
an output line 40 for communicating with other facilities as
desired. In this regard, the RS 232 communication line can be used
by engineers to load or revise software relating to the operation
of the DSP. The system may also include a CAN or centralized area
network bus 42 that is essentially a serial bus that is connected
to the DSP via line 44 and it has output line 46 for communicating
with the outside world. It could, for example, report all of the
parameters that the DSP was measuring and forward such data for
diagnostic purposes. The RS 232 as well as the CAN bus circuitry
are also conventional and are therefore not described in
detail.
As previously mentioned, the preferred embodiment of the present
invention has a dual functionality in that it produces a high
voltage pulse that is applied to the igniter plug which causes it
to ionize and discharge and that event is closely followed by a
high energy current being applied to the igniter plug. Referring to
the block diagram, the high energy 650 volts flyback charger 22 is
controlled by the DSP 20 via line 52. The flyback charger 22 is
also connected to a low energy 400 VDC passive charger circuit 56
by a line 54 and to a high energy capacitor located in a high
energy ignition circuit 58 by a line 60. The charger 56 has output
line 62 that extends to a low energy ignition circuit 64 which
contains the high voltage step-up transformer and low energy
capacitor. The low energy ignition circuit 64 is connected to the
high energy ignition circuit 58 via line 66.
The charge on the low energy capacitor in circuit 64 as well as the
high energy capacitor in circuit 58 is provided to a voltage
feedback circuit 68 through line 70 and 62 and the voltage feedback
circuit 68 provides signals on line 72 to the DSP 20 for
determining when both the high energy capacitor and the low energy
capacitor are charged to their predetermined levels. While the
specific circuitry that implements this portion of the block
diagram will be described in detail, the operation essentially
comprises the DSP providing a signal on line 52 to the flyback
circuitry 22 which causes it to turn on and begin to charge up the
low energy capacitor in block 64 as well as the high energy
capacitor in block 58. As both capacitors are charging, they
provide signals on respective lines 62 and 70 that is reported back
to the DSP via line 72. When both capacitors reach their
predetermined charge value, which takes approximately 300
milliseconds, the DSP provides a signal to the circuit 52 to stop
charging. When both capacitors are charged to their desired energy
value, the DSP then fires the SCR switches in block 64 and 58 in
their proper timed sequence and ignition occurs. More particularly,
the DSP 20 initiates firing of the circuit by initially triggering
the switch which releases the energy from the high energy capacitor
bank with that signal being applied by the DSP 20 on line 76,
followed by triggering of the switch that discharges the low energy
capacitor in circuit 64 with the trigger signal being applied on
line 74.
The feedback functionality also enables the DSP 20 to perform
diagnostic operations utilizing the monitored values that it
receives. For example, if the ignition system is fired and a
millisecond later the DSP 20 detects that there is still a large
voltage on the capacitors, the DSP can conclude that there was a
malfunction in the firing circuitry or that the igniter plug was
either dead or missing.
It should also be understood that the output signals from the DSP
are typically in the range of 3 volts and are very low power
signals. Since the SCR switches need to be driven with a much
larger signal, it should be understood to one of ordinary skill in
the art that conditioning and converting circuitry is necessary to
interface the signals from the DSP 20.
Turning now to the specific circuitry of the high energy ignition
circuitry 58 and the low energy ignition circuit 64, and referring
to FIG. 2, the portion to the left of the vertical dotted line
illustrates the low energy ignition circuit whereas the portion to
the right of it represents the high energy ignition circuitry 58.
Line 100 is connected to the low energy capacitor 102 and to the
primary winding of a step-up transformer 104 as well as to the
cathode of a diode 106. The anode of the diode 106 is connected to
line 110 that is also connected to the primary winding of the
transformer 104 and to the anode of an SCR 112, the cathode of
which is connected to ground 114. Diode 108 is connected "anti
parallel" with SCR 112. A gate terminal 116 is connected to the DSP
through conditioning circuitry that provides sufficient power to
place the SCR 112 into conduction rapidly once it is triggered.
The secondary winding of the transformer 104 is connected to line
118 that extends to one terminal of an igniter plug 120, the other
terminal of which is connected via line 122 to ground as well as to
one terminal of a capacitor bank 124 having three parallel
connected capacitors 126. The opposite side of the capacitor bank
has line 128 connected to a pair of SCR's 130 and 132. Respective
gate terminals 134 and 136 are connected to the DSP 20 through
suitable conditioning circuitry to provide the proper energy level
at the gates of the SCR's to rapidly place them into full
conduction. The cathodes of the SCR's 130 and 132 are connected to
respective inductors 138 and 140 which are in turn connected via
line 142 to the secondary winding of the transformer 104 as well as
to a number of series connected diodes 146 and a number of series
connected resistors 148 that are individually connected in parallel
to an associated diode. The diodes 146 are also connected in
parallel with the secondary winding of the transformer 104 in
addition to being in parallel with the resistors 148. It should be
understood that the SCR's 130 and 132, while shown to be connected
in parallel, could be series connected, and the series connected
diodes 146 could also be parallel connected.
With regard to the low energy ignition circuit, the low energy
capacitor 102 is charged to a voltage of approximately 400 volts DC
by the passive charge circuit 56 (not shown in FIG. 2). The low
energy capacitor has an energy capacity of less than 2 Joules and
is preferably about 300 millijoules. (approximately 4 microFarads)
which provides the energy for generating the high voltage pulse at
the output line 118 when the low energy capacitor is discharged
through the primary winding of the transformer 104. This occurs
when the DSP generates a pulse that is conditioned and applied to
the gate terminal 116 of the SCR 112. When the SCR 112 is gated
into conduction, the current from the capacitor 102 flows through
the primary winding and by virtue of the ratio of windings from the
primary to secondary, produces an open circuit voltage up to
preferably between approximately 15,000 and approximately 20,000
volts in the secondary which appears on line 118 and is applied to
the igniter plug 120. In this regard, the voltage may be within a
larger range of between 1,000 and 50,000 volts and still be
functionally operable, but the approximately 15,000 to
approximately 20,000 volt range is known to produce reliable
operation.
The DSP 20 turns on the high energy 650 volt flyback circuit 22 to
charge the capacitor bank 124 to a voltage of preferably about 650
volts. After the capacitor 124 is charged, the DSP 20 produces a
trigger signal on line 76 which is conditioned by circuitry (not
shown) to provide a robust gate signal to gate terminals 134 and
136 to switch the SCR pair 130, 132 into conduction. It is
important to place the SCR's 130 and 132 in conduction quickly so
that the current from the capacitor 124 does not damage the SCR's.
In this regard, the capacitor bank 124 has an energy capacity of
less than 20 and preferably approximately 16 Joules so that when
the SCR switches 130 and 132 are triggered into conduction, a
current flow of approximately 1,000 to 2,000 amperes is
produced.
The energy is conducted through the SCR's into saturable reactors
138 and 140. These saturable reactors are included for the purpose
of protecting the SCR's from damage due to excessive current flow
and also to ensure current sharing between the parallel connected
SCR's. The current limiting function, which is preferably only
approximately 4 to 5 microseconds, but which may be within the
range of approximately 1 to approximately 10 microseconds, gives
the SCR's time to bring sufficient area of their structure into
conduction before high current starts to flow. After the very short
delay, the high rate of change of current, di/dt, is permissible
without causing damage to the SCR's. This is particularly useful
under ignition lead faults which would result in very high peak
currents with very high di/dts. Additionally, the impedance of the
saturable reactor after saturation helps share the high energy
current between the parallel connected SCR switches. The current
then flows through line 142 to the series connected diodes 146
which are connected in parallel with the secondary winding of the
transformer 104 and the high current is conducted to line 118
through these multiple diodes 146.
Because the voltage that is generated by the high voltage pulse is
up to 20,000 volts, the four diodes 146 that are utilized are rated
at 5000 volts each. These are relatively expensive diodes, but are
necessary to the proper operation of the system. The use of the
resistors 148 in parallel insure that the voltage of each diode is
shared more or less equally. It should be appreciated that there is
a significant heat loss in these high voltage diodes because high
voltage diodes typically have a lot of resistive loss when they are
conducting current. With current levels in the range of 1,000 to
2,000 amps being conducted through the diodes 146, they tend to
become relatively hot. By using four 5,000 volts diodes, the heat
generated is spread among four semiconductor diodes.
During operation and referring to FIG. 3, the DSP 20 initially
triggers the SCR's 130, 132 when the capacitor bank 124 and the low
energy capacitor 102 are charged to their respective voltages of
650 and 400 volts. When the SCR's are placed into conduction at a
particular time, (FIG. 3a) then preferably approximately 5 to 7
microseconds later, the SCR 112 is gated into conduction as shown
in FIG. 3b. In this regard, it should be understood that the delay
between triggering the SCR's 112 and 130 may be within the range of
approximately 0.1 to approximately 10 microseconds. The voltage on
SCR 130, 132 is initially at 650 volts but quickly declines to 0 in
approximately 1 microsecond as shown in FIG. 3c. The conduction
area of the SCR 130 and 132 gradually ramps up in 5 to 10
microseconds and is then conditioned for high rates of current flow
as illustrated in FIG. 3d. As shown in FIG. 3e, the voltage applied
to the plug 120 starts at 0 and increase to 650 volts when SCR 130
is gated in conduction and maintains that voltage level until the
SCR 112 fires causing the high voltage pulse of up to about 15,000
to about 20,000 volts to be generated which creates ionization and
breakdown of the plug 120, placing it into conduction (typical
breakdowns may be between 1 and 5 kV). The reactor voltage
transitions from 0 to about 650 volts when breakdown occurs and it
limits current flow until the saturable reactor saturates which
requires approximately 5 microseconds whereupon the rate of current
rise increases dramatically as shown in FIG. 3g.
The diode 106 is a freewheeling or flyback diode that is often
included as a matter of standard practice. Whenever there is an
inductive load such as an ignition coil or the primary winding of
the transformer 104 in the illustrated circuit, when the SCR 112
opens, there is still current flowing in the primary coil of the
transformer and the energy has to be conducted to some destination
or a very high voltage spike will be produced. Its presence insures
better reliability.
On the high energy side of the circuitry, a diode 150 is provided
as a clamping diode which also provides a path or current flow
after the plug has been fired. This device keeps the capacitor bank
from seeing a high negative voltage as the igniter current passes
through 0. In prior art designs this clamping diode saw high
current levels for a large percentage of the energy discharge
because the underdamped discharge characteristics were dominated by
a wave shaping inductor. The proposed low voltage, high capacitance
system does not conduct appreciable current through this clamping
diode, because the higher capacitance values associated with a low
voltage system (124) provide for more damping in the RLC discharge
network.
Further advantages of the low voltage, high capacitance system
relative to the prior art high voltage, low capacitance systems are
as follows. The discharge characteristics in a high capacitance
system are dominated by the capacitor. If the external conditions
place more resistance between the exciter and the igniter, the peak
current decreases while the current duration increases. The
decreasing peak current tends to decrease the energy delivery to
the igniter while the increasing current duration tends to increase
the energy delivery to the igniter. They tend to cancel each other
out and reduce the variation in total energy delivered to the
igniter as a function of external resistance. In contrast, the
prior art, low capacitance, unipolar systems discharge their
capacitors relatively instantaneously and rely on a wave shaping
inductor to provide energy to the igniter during the majority of
the discharge. If the external conditions place more resistance
between the exciter and the igniter, the peak current decreases
while the current duration also decreases. Both of these reductions
decrease the energy delivered to the igniter. Thus, the low
capacitance, unipolar systems have a higher variation in total
energy delivered to the igniter as a function of external
resistance relative to a high capacitance system.
From the foregoing discussion, it should be appreciated that an
ignition system has been shown and described which has many
desirable attributes and advantages. The system advantageously
utilizes a low energy ignition circuit and transformer to provide a
very high voltage pulse that is applied to the igniter plug 120 and
produces ionization and breakdown before the energy from a high
energy capacitor bank is applied to sustain the spark initially
produced by the high voltage pulse. The unique design of the system
does not subject the step-up transformer that generates the high
voltage pulse to the very high current flow that originates with
the high energy capacitor. Importantly, the use of a low voltage
bus in the high energy ignition circuit portion of the system
results in advantageous use of less expensive semiconductor devices
and yet produces a highly reliable and effective ignition
system.
While various embodiments of the present invention have been shown
and described, it should be understood that other modifications,
substitutions and alternatives are apparent to one of ordinary
skill in the art. Such modifications, substitutions and
alternatives can be made without departing from the spirit and
scope of the invention, which should be determined from the
appended claims.
Various features of the invention are set forth in the following
claims.
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